Engines may incur cylinder torque imbalances due to various reasons. These may include, for example, blocked injector nozzle holes, over-boost, fuel quality issues, and soot accumulation in cylinder. Engine control systems may be configured to address cylinder torque imbalances to improve engine performance.
One example approach for reducing cylinder imbalances is shown by Yamaoka et al. in U.S. Pat. No. 7,128,048. Therein, cylinder imbalances are identified based on deviations in cylinder pressure peak timings from a predetermined timing range. Based on whether the cylinder pressure peak timing in a given cylinder is retarded or advanced relative to the predetermined range, an engine controller may adjust an amount of internal EGR delivered to the affected cylinder. This allows an ignitability of the mixture in the cylinder to be improved, and an engine speed to be appropriately adjusted. If the deviation is sufficiently large, a compression ignition mode of engine operation may be inhibited to reduce engine degradation.
However, the inventors herein have recognized potential issues with such an approach. As an example, adjusting the amount of internal EGR may not reduce cylinder imbalances caused by hydrocarbon accumulation in an engine intake. For example, in some engine system, certain engine cylinders may be more prone to hydrocarbon accumulation due to the specific configuration of the engine intake system. Herein, increasing the amount of internal EGR may not help to release the hydrocarbons from those cylinders. As another example, hydrocarbons may accumulate at a charge air cooler and be forced from there into the engine during a tip-in. Increasing an amount of internal EGR may reduce the power provided during the tip-in without improving release of hydrocarbons from the charge air cooler. If left at the engine intake, the accumulated hydrocarbons may eventually spread from the affected cylinder to remaining cylinders, causing further cylinder imbalances and expediting engine degradation.
Thus in one example, some of the above issues may be addressed by a method of operating an engine comprising in response to cylinder imbalance and an elevated engine exhaust exotherm indicative of hydrocarbon oxidation, limiting engine speed and load to reduce hydrocarbon accumulation at an engine intake. In this way, hydrocarbon accumulation at one or more locations along an engine intake can be better addressed.
In one example, an engine may include a branched intake system providing air to each of a first and second group of cylinders. Due to the specific configuration of the intake system, air may flow from a throttle to a y-junction, and then from a first outlet of the y-junction to the first group of cylinders and from a first outlet of the y-junction to the second group of cylinders. A longitudinal axis of the first outlet of the y-junction may be aimed at a first cylinder positioned away from an end cylinder of the first group while a longitudinal axis of the second outlet of the y-junction may be aimed at the first cylinder positioned away from an end cylinder of the second group. Consequently, the first cylinder of the first group and the first cylinder of the second group may be more prone to hydrocarbon accumulation. An engine controller may determine cylinder imbalances based on crankshaft acceleration differences estimated during steady-state engine operating conditions (e.g., idling conditions) and/or transient engine operating conditions (e.g., during a tip-in). The crankshaft data may be estimated in different windows during steady-state conditions and transient conditions, the window varying based at least on the mass air flow during the respective condition. Due to higher background noise, the controller may perform significant signal processing of the crankshaft data received during the transient conditions, including debouncing of the signals, to differentiate cylinder imbalances arising due to engine intake hydrocarbon accumulation from cylinder imbalances arising due to air or fuel variations (e.g., from misfires) during the transient conditions. In addition, during conditions when an exhaust particulate filter is not regenerating, the controller may estimate exhaust temperature differences across an exhaust catalyst (such an exhaust oxidation catalyst).
In response to a cylinder imbalance detected while an exhaust exotherm is elevated, the controller may determine that there is oxidation of hydrocarbons that were accumulating at an engine intake. In particular, uncontrolled hydrocarbon accumulation may have occurred at various locations along the engine intake including at the crankcase, near the intake port of specific cylinders, and at or near a charge air cooler. In response to the indication, the controller may limit engine speed and load to reduce further hydrocarbon accumulation at the engine intake. The limiting may include limiting fuel injection to all engine cylinders including the imbalanced cylinder. The degree of limiting may be based on whether the cylinder imbalance was detected during transient conditions or steady-state conditions. For example, in response to cylinder imbalances and an elevated exotherm detected during transient conditions, the limiting may be higher and faster as a result of high Alpha and Beta error separation while in response to cylinder imbalances and elevated exotherms detected during steady state, the limiting may be lower and slower due to smaller Alpha and Beta error separation, requiring slower debounce rate. The controller may also elevate engine temperatures so as to release the accumulated hydrocarbons. Herein, the controlled oxidation or evaporation of the accumulated hydrocarbons increases tolerance to crankcase overfilling. Further still, the controller may set one or more diagnostic codes, illuminate an indication light, and set a cluster message to alert the vehicle operator that hydrocarbon accumulation at the intake was detected, so that the vehicle operator can take the vehicle to a service center before substantial engine degradation can occur.
In this way, by using crankshaft acceleration differences to identify cylinder imbalances during steady-state and transient engine operating conditions, and by correlating the cylinder imbalances with elevated exhaust exotherms, cylinder imbalances due to hydrocarbon accumulation at the engine intake can be better identified and distinguished from cylinder imbalances caused from hydrocarbon accumulation at other engine locations, and cylinder imbalances due to other engine conditions (e.g., those due to fuel injector variations). By limiting engine speed and load responsive to the indication of hydrocarbon accumulation at the engine intake, primary engine degradation caused by flow of the accumulating hydrocarbons into the engine during high air flow conditions (such as a tip-in), can be reduced. In addition, by limiting further accumulation of hydrocarbons at the engine intake, secondary engine degradation that could potentially arise from continued cylinder imbalances can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.